1. Field of the Invention
The present invention relates to an image fading preventing agent, an image forming element, a recording medium, an image forming method and an image using the same.
2. Description of the Related Art
In recent years, the advancement of image forming technology has rapidly progressed, an image quality comparable with that of a silver halide photograph has been achieved even in image forming methods such as a dye sublimation thermal transfer method and an ink-jet method, and such high quality image spreads in the life of the people.
The dye sublimation thermal transfer method is a method in which sublimation coloring materials are used as recording agents, these agents are carried on a base sheet such as a polyester film to provide a thermal image transfer sheet (ink sheet), and various kinds of full-color images are formed on an image receiving sheet (recording medium for thermal transfer) obtained by forming a coloring material receiving region on a transfer printing medium capable of being dyed by the sublimation coloring material, for example, paper, a plastic film or the like. In this case, a blinking thermal head is used as a heating means to transfer many color dots of three colors or more to the image receiving sheet by heating for an extremely short period of time, thereby reproducing a full-color image of an original image with the multi-color color dots. Concerning the image formed in such a manner, an image obtained by using a dye as a sublimation coloring material can be excellent in brightness, transparency, half tone reproduction property and gradation property and having high quality comparable with a full-color silver halide photograph image.
However, the image obtained by using sublimation dyes is generally poor in light fastness compared with an image formed with pigments and involves a problem that the image is fast faded or discolored when it is exposed to direct sunlight. The image also involves a problem that it undergoes discoloration/fading (dark fading) even when it is exposed to no direct sunlight, for example, when it is in a room, placed in a file or kept in a book. These problems of light fastness and dark fading are improved to some extent by adding an ultraviolet light absorber and an antioxidant to a dye receiving layer of the image receiving sheet. However, the above-described conventional technique involves a problem that the effect to protect the dyes by the antioxidant and the like is not effectively achieved and the prevention of discoloration/fading is insufficient, because the antioxidant and the like are uniformly distributed in the whole of the dye receiving layer, while most of the dyes transferred are present in the vicinity of the surface of the receiving layer. There is thus a strong demand for development of a technique capable of effectively protecting the transferred dyes by the antioxidant and the like.
On the other hand, ink-jet recording has been rapidly spread for output apparatus of digital signals from computers, digital cameras, etc. in recent years because it has features of low price of an apparatus itself, low running cost, easy color recording and the like. As for recording inks used for ink-jet printers, there have been proposed various types of inks such as inks obtained by dissolving a dye in an aqueous medium or non-aqueous medium, inks obtained by dispersing a pigment in an aqueous or oil medium, and heat-fusible solid inks. Those obtained by dissolving a dye in an aqueous medium are recently mainly used for such ink and have features of beautiful coloring, high safety for human body and environment, and the like. In recent years in particular, ink-jet printers capable of conducting printing with ease in homes and offices have been spread, and those capable of attaining image quality comparable with that of a silver halide photograph have been developed. However, among coloring materials such as aqueous dyes, there are those poor in light fastness and oxidation resistance. Therefore, an image formed by using such dyes is poor in shelf life compared with the silver halide photograph and particularly involves a problem that its image quality and hue are deteriorated by light or the like with time. In the ink-jet printing, there is also a strong demand for development of a technique capable of effectively protecting the dyes by the antioxidant and the like.
In order to solve such a problem, there have been proposed various solving methods of preventing degradation of dyes. The methods mainly comprise adding an antioxidant, an ultraviolet light absorber and/or the like and providing an overcoat thereof, and proposals have been made as to the kinds of additives, adding methods, properties of the antioxidant, etc.
There are various theories as to the mechanism of fading of coloring materials such as dyes usable in the dye sublimation thermal transfer recording or ink-jet recording. However, it is generally considered as follows. Namely, a coloring material such as a dye becomes in the triplet excited state by light, and excited singlet oxygen is generated by energy transfer between the coloring material in the triplet state and triplet oxygen in the ground state. This singlet oxygen is high in activity and relatively long in life and reacts with the coloring material such as the dye to facilitate oxidation or decomposition, which is considered to form the cause of deterioration of an image.
From such a point of view, there have been known those relating to additives with a singlet oxygen quenching rate kQ defined as a parameter for a method for selecting a suitable additive, and recording media and recording liquid containing such an additive, or recording methods using the same (Japanese Patent Application Laid-Open No. 2000-226544 (Patent Document 1) and Japanese Patent Application Laid-Open No. 2003-94797 (Patent Document 2).
Patent Document 1 discloses recording liquids for ink-jet printing and recording sheets for ink-jet printing containing a compound having a singlet oxygen quenching rate of 1×106 dm3mol−1s−1 or more. Patent Document 2 discloses ink-jet recording media using a compound having a singlet oxygen quenching rate of from 1×106 dm3mol−1s−1 or more to 1×109 dm3mol−1s−1 or less and a compound having a radical trapping rate (Ks) of from 1 or more to 10,000 or less in combination.
Japanese Patent Application Laid-Open No. 2000-34433 (Patent Document 3) discloses ultraviolet dissipation energy absorbers as additives for improving fading resistance of printed images, and binding amine free radical inhibitors (hindered amine compounds) and reducing agents (hindered phenol and the like) as other additives.
In Journal of Physical Chemistry Reference Data, Vol. 10, No. 4, pp. 809-999 (1981) (Non-patent Document 1), arguments on the results of measurement of the singlet oxygen quenching rate kQ of various kinds of compounds are made for every solvent.